Solutions-Class 7-Science-Chapter-20-In the World of Star-Maharashtra Board

(a) When seen from a great distance, the sky seems to be touching the ground along a circle. This circle is called the horizon.

(b) The ecliptic is used while defining the zodiac signs.

(c)  Classified according to seasons, one season will have nine nakshatras.

(d) The rising of the sun in the east and its setting in the west is the apparent motion of the sun.

The star will rise at 6 pm after a month. This is because each star rises or sets 4 min early every next day. Therefore, after a month, the total advancement in the rising time of a star = 4×30=120 min=2 h Thus, if a star rises at 8 pm today, it will rise before 2 h i.e. at 6 pm after a month.

As the earth changes its position due to its revolution, a different constellation or raashi appears behind the sun every 18° 20’. But this is expressed as if sun has entered a particular raashi. So, when we say sun has entered Mrug nakshatra, actually Mrug constellation is behind the sun.

A group of stars occupying a small portion of celestial sphere is called a constellation. Some of these stars appear to form certain animal or human figures and objects. These constellations are known by the brightest star present in it. Few examples of constellations are Orion, Big Dipper.
 

• All the equipment such as good quality binocular, telescope, the sky map, the compass to show directions are to be gathered before sky observation.
• The place chosen for sky watching should as far as possible from the city.
• The night chosen for sky watching should be new moon night.
• Pole Star should be chosen as a reference point for sky watching.
• Sky watching should begin with stars in west.

(Students should write answer to this question based on their own experiences.)

• Distant stars, planets, comets or any constellation do not have any influence on human life.
• In the age of scientific and technological progress, man has stepped over the moon.
• Till now there is no credible evidence supporting astrology.
• Therefore, it would certainly be wrong to say that planets, stars and nakshatras affect human life.
• Science, as we know, does not follow beliefs but has its basis on observations and evidences.
• Therefore, unless it is scientifically proved, we cannot say that stars, planets and celestial bodies have impact on human life.

• The stars are born out of nebulae which are the clouds of hydrogen gas and dust particles.
• In the nebulae there are particles that are attracted towards one another due to the force of gravity.
• This makes contractions in the clouds resulting into denser and spherical bodies.
• At the same time, at the core of clouds the pressure of the gas increases. This causes rise in the temperature to tremendous extent. The energy is generated due to this condition.
• This spherical cloud of hydrogen is called a ‘star’.

The processes such as contraction, expansion, rise in temperature, etc. bring about changes in the nature of the star eventually. These changes are very gradual and spread over a very long period of time. They constitute the lifecycle of stars.

Other stages of a star during its life cycle is
Sun-like Stars: These are particularly similar to the sun in size but there is a lot of variation in terms of temperature. Examples are Alpha Centurai, Tau Ceti etc.
Red Giants: These stars do not have as hot outer layer as that of the Sun with temperature ranging from 3000 ^oC to 4000 ^oC but they have a very high luminance than the Sun. They are red in color and their diameter is 10 to 100 times that of the sun.
Super Nova: They are larger and even brighter as compared to the red giant stars and even the Sun. They are also considered as the primary source of heavy elements in the universe.
Binary or Twin Stars: A pair of two stars in which one revolves around the other or they both revolve around a common center is known as binary or twin stars. 
Variable Stars: These stars have a variable shape and brightness i.e it keeps on changing whenever they expand or contract. Their brightness decreases when they expand and increases when they contract. Example Polaris or Pole Star.